Hydrogen cyanide, HCN, is a source of valuable compounds used in many industrial processes, e.g. KCN, NaCN and Ca(CN).sub.2. When solutions or waste streams contain cyanide compounds other than HCN, such as metallic complexes, it is often useful to convert these compounds to the more convenient salts which can be sold or used directly.
A further benefit of recovery of cyanide values from solution arises from restrictions on the disposal of cyanide compounds. Processes which produce cyanide compounds as a waste product must include some method for disposing of these cyanide compounds in an environmentally acceptable manner. Alternative solutions to the cyanide removal problem as applied to gold mill effluents are described in Scott and Ingles, "Removal of Cyanide from Gold Mill Effluents," Paper No. 21 of the Canadian Mineral Processors Thirteenth Annual Meeting, Ottawa, Ontario Canada, Jan. 20-22, 1981.
The cost of disposal can be somewhat offset if the cyanide values can be recovered for recycle in the process itself or for sale to another user. However, many of the most common cyanide removal techniques involve oxidation whereby the cyanide is destroyed. Moreover, recovery of useful cyanide from waste streams has proved to be particularly difficult when, as is typically the case, the cyanide is present in only low concentrations in the waste stream, e.g. on the order of less than about 0.5 percent by weight. Recovery of cyanide values is further complicated by the presence of metals in the waste stream and particularly by the tendency of cyanide to form complex ions with metals, such as Fe, Ni, Co, Zn and Cu.
One method which has been used to recover HCN from a cyanide-containing solutions is acidification/volatilization/reneutralization (AVR). This process takes advantage of the very volatile nature of hydrogen cyanide at low pH. In the AVR process, the waste stream is first acidified to dissociate CN.sup.- from metal complexes and to convert it to HCN. The HCN is volatilized usually by the introduction of steam, often accompanied by air sparging. The HCN evolved is then recovered, for example, in a lime solution and the cyanide-free wastestream is then reneutralized. A commercialized AVR method known as Mills-Crowe method is described in the Scott and Ingles paper. One difficulty with known AVR processes is that when a solution contains only low concentrations of cyanide compounds, the reagent costs for acidifying the stream and later neutralizing the waste solutions and/or the energy costs associated with raising the solution temperature to achieve volatilization become extremely high compared to the benefit of recovering the cyanide values.
As discussed in Ingles and Scott, "Overview of Cyanide Treatment Methods for Presentation at the Cyanide Cold Mining Seminar," Ottawa, Ontario, Canada, Jan. 22, 1981 and in U.S. Pat. Nos. 4,267,159 and 4,321,145, another method for cyanide removal from waste streams involves use of ion exchange resin beds. A number of resins, both weak and strong base, are known to be selective for both free and complexed cyanides. See for example, U.S. Pat. Nos. 3,984,314; 3,788,983; 4,267,159; 4,321,145; and 4,115,260. One problem associated with the use of strong base resins is the difficulty in eluting the cyanide once it is absorbed. Certain cyanide complexes, e.g. cyanide complex of Zn and Cd, are difficult to elute even from commercially available weak base resins.
U.S. Pat. Nos. 3,391,078 and 2,507,992 each disclose methods in which Ca(OH).sub.2 is used as a regenerant for certain ion exchange resins from which at least some cyanides can be eluted with hydroxides. Although lime or Ca(OH).sub.2 is economically preferred over NaOH as a reagent, its low solubility in water generally makes its use more difficult than more soluble hydroxides, e.g. NaOH.
While U.S. Pat. No. 4,321,145 suggests an AVR method to recover HCN from the cyanide loaded resin eluate, its teachings are limited to the use of a complex multi-level resin bed having a strong base anion exchange resin layer, a weak acid cation exchange resin and a strong acid cation exchange resin and the concomitant need for a complex resin regeneration sequence.
Accordingly, it is an object of the present invention to provide a novel method for recovering rather than destroying cyanide values from streams initially containing relatively low levels of cyanide, using weak base ion exchange resins to concentrate the cyanide before recovery.
Another object of the present invention is to provide an improved method of eluting complex cyanides, such as complexed zinc cyanide, from weak base resins.
Yet another object is to provide a unique system of concentrating cyanide in the eluant of an ion exchange resin using calcium hydroxide as the resin regenerant. A further object is to provide a novel calcium hydroxide elution circuit which permits full elution of cyanide values using a predetermined volume of eluant so as to concentrate the cyanide therein.
Still another object is to provide a method whereby acidification/volatilization techniques can be used to recover cyanide from low level cyanide streams due to effective concentrating of the cyanide values prior to acidification.
These and other advantages are achieved by practice of the processes of the present invention as described hereinbelow.